Q.

What is the freezing point depression constant of a solvent, 50 g of which contain 1 g non volatile solute (molar mass 256 g mol-1) and the decrease in freezing point is 0.40 K            [2025]

1 5.12 K kg mol-1  
2 3.72 K kg mol-1  
3 4.43 K kg mol-1  
4 1.86 K kg mol-1  

Ans.

(1)

ΔTf=mkf 

ΔTf=wsolute(in g)×1000MsoluteWsolvent(in g)kf 

0.4=1×1000256×50kf 

kf=0.4×256×501000K kg mol-1

=5.12 K kg mol-1